Well, if you count history, then it's Madrid no doubt.
But as of now, there are many contendors and it's hard to tell. You've got strong attacking teams like United, Bayern, Liverpool, and Milan.
But every team makes mistakes so it's hard to tell.
United = FA Cup Defeat.
Bayern = Only in UEFA Cup this year.
Liverpool = Were struggling to make the top 4.
Milan = Not even close to winning the title and early exit in the CL.
There's no "best team in the world out there". You can have a series between teams and it will result in losses/wins, but it will be pretty even. In a few years, Arsenal will be up there because their players are growing, same with United, look at what an aging squad we have. Ronaldo, Rooney, Nani, Tevez, Anderson, Pique, Foster, Carrick. All these players have tons of years to look to.